I need a new tranny on my truck, my big mower's in the shop, and now my backup mower is having issues! I had to push a yard that was three feet high today! Here's the deal; my backup shuts off when I try to engage the blades or even go forward or backward. My guess is that there's s/t wrong with the safety shutoff. It's a JD 318, garden tractor type. Any ideas??